The present invention relates to an improved sawtooth voltage generator having a wide frequency variation range which is particularly useful for phase triggering control purposes.
Sawtooth voltages are generally produced by slowly charging a capacitor with a constant current and then rapidly discharging the capacitor via an electronic switch. The switch is preferably formed by the collector-emitter path of a transistor.
In a known sawtooth generator of this type (e.g. see the book by Tietze and Schenk, "Halbleiter-Schaltungstechnik" [Semiconductor Switching Art], 1971, pages 568-570) the switch is constituted by a pair of transistors which are interconnected to form a simulated thyristor or a unijunction transistor. Such a pair of transistors permits particularly rapid discharging of the capacitor.
In practice, there often exists the desire to vary the frequency of such a sawtooth generator. The frequency is determined by the value of the capacitor as well as by the charging current. However, the charging current in such known circuit can be varied only within relatively narrow limits because it must not fall below the firing current for the transistors nor exceed the holding current for the transistors.
Such a sawtooth generator is shown in FIG. 1 and includes a capacitor 5 having one terminal connected to ground and its other terminal connected to the output of a constant current source. The capacitor 5 is charged with a constant charging current I from the current source 6 so that the voltage V.sub.S at terminal 7 increases in a sawtooth manner in the negative direction. Connected to the capacitor 5 is a pair of complementary transistors 1, 2, which constitutes a simulated thyristor or a unijunction transistor and which serves to suddenly discharge the capacitor 5. The transistor 1 has its emitter connected to one terminal of the capacitor 5, its collector connected to the base of the transistor 2, and its base connected to the collector of the transistor 2 and to a point of reference or threshold voltage V.sub.1 provided at the output of a voltage divider formed by a Zener diode 8 and a resistor 9 connected in series between the input terminal for a source of supply voltage -V.sub.B and ground. The emitter of transistor 2 is connected to ground. In the illustrated embodiment of the sawtooth generator, in view of the polarity of the supply voltage -V.sub.B and the current I, the transistor 1 is an NPN transistor and the transistor 2 is an PNP transistor. It is to be understood, however, that if desired the circuit may be operated with a supply voltage of the opposite polarity in which case the polarity type of the transistors 1 and 2 would be reversed.
During the charging process of capacitor 5, the transistors 1 and 2 are normally nonconductive. If the voltage V.sub.S has reached the value -(V.sub.1 + V.sub.BE1), where V.sub.BE1 is the base-emitter voltage of transistor 1, transistors 1, 2 become conductive and cause a rapid discharge of capacitor 5. After the sudden discharge of capacitor 5 transistors 1 and 2 become nonconductive and the charging process from current source 6 begins anew.
The charging current I must here exceed a minimum value, i.e., firing current I.sub.Z, in order for transistor 2 to be fully switched through, i.e., conductive. However, with very low currents, the current amplification of the transistors 1 and 2 decreases. Moreover, the current amplification factor of transistor 2 is additionally reduced in that a discharge resistor 10 must generally be connected between the base and the emitter of transistor 2 in order to discharge the blocking current of transistor 1. If now the charging current I is not sufficient to produce the necessary voltage drop across resistor 10 to switch through transistor 2, the voltage V.sub.S may get stuck and remain at the value -(V.sub.1 + V.sub.BE1). The charging current I must thus not be reduced to a value below I.sub.Z during setting of the frequency.
In the switched-through state of the transistors 1 and 2, current additionally flows through transistor 2 via voltage divider resistor 9. The discharge current from capacitor 5 and the charging current I now keep transistor 2 conductive. If, however, capacitor 5 has been discharged to the value V.sub.BE1 + V.sub.CES2 (collector-emitter saturation voltage of transistor 2) the charging current I CES2 no longer be sufficient to keep the transistor circuit conductive so that the circuit will be switched as desired. This also produces an upper limit value for the charging current I, i.e., the holding current I.sub.H, which when exceeded causes transistors 1, 2 to remain conductive. As soon as the firing voltage has been reached, V.sub.S will collapse to the above-mentioned value and remain stuck there.